1. Make topping: Mix oats, flour, dark brown sugar and salt in a large bowl. Using fingertips, a pastry blender or two knives scissor-fashion, cut in butter until pieces are size of peas and mixture is well combined. Keep refrigerated until ready to use.
2. Preheat oven to 375°F; butter a 9-by-13-inch baking dish. Make filling: Combine nectarines and blueberries in a large bowl. Add lemon juice, sugar, cornstarch and cinnamon. Mix well and pour into baking dish.
3. Sprinkle topping mixture evenly over fruit and bake until brown on top and fruit is bubbling, about 45 minutes. Let cool on a wire rack before serving.